ch15

15 15-1 Adsorption Introduction to Adsorption Phenomena Adsorption Phenomena Historical Development Applications of Adsorption Materials 15-2 Manufacture, Regeneration, and Reactivation of Activated Carbon Manufacture from Raw Materials Regeneration and Reactivation of Spent GAC 15-3 Fundamentals of Adsorption Interfacial Equilibria for Adsorption and Other Solute Surface Phenomena Important Factors Involved in Adsorption Surface Chemistry and Forces Involved in Adsorption 15-4 Development of Isotherms and Equations Used to Describe Adsorption Equilibrium Equilibrium Isotherm Langmuir Isotherm Equation Freundlich Isotherm Equation Brunauer–Emmett–Teller Isotherm Equation Polanyi Correlation for Liquids Multicomponent Equilibrium Dubinin–Radushkevich Correlation for Air Stripping Off-Gases 15-5 Powdered Activated Carbon Uses of PAC in Water Treatment Experimental Methods for Determining PAC Dosages Comparison of Carbon Usage Rates for PAC and GAC Factors That Influence PAC Performance Use of PAC in Unit Operations Homogeneous Surface Diffusion Model 15-6 Granular Activated Carbon Terms Used in GAC Application Determination of Specific Throughput and Carbon Usage Rate MWH’s Water Treatment: Principles and Design, Third Edition John C Crittenden, R Rhodes Trussell, David W Hand, Kerry J Howe and George Tchobanoglous Copyright © 2012 John Wiley & Sons, Inc 1117 1118 15 Adsorption GAC Operation Modeling GAC Performance Evaluating the Impact of Natural Organic Matter on GAC Performance Rapid Small-Scale Column Tests Factors That Impact Adsorber Performance Problems and Discussion Topics References Terminology for Adsorption Term Particle properties Adsorbent Adsorbent particle density in an adsorber, ρs Apparent particle density, ρa Particle porosity, εp Sphericity, φ Specific surface area Adsorber properties Bed porosity, ε Definition Solid media on which adsorption occurs Weight of the dry (and fresh) adsorbent particles divided by the solid volume The density of activated carbon is approximately equal to the density of graphite (≈2 g/mL) Weight of the dry (and fresh) adsorbent particles divided by the total volume of the adsorbent particle The total volume includes the solid and pore volume Ratio of the pore volume to the total volume of an adsorbent particle This parameter characterizes the fraction of the adsorbent volume that is not occupied by the carbon material εp = − (ρa /ρs ) External surface area of a particle divided by the surface area of a sphere that would have the same volume Describes the increase in surface area due to a particle having an irregular shape External surface area per weight of a dry particle Because most adsorbent particles have an irregular shape, the external surface area per unit mass is defined as 3/Rφρa where R is equal to particle radius Void volume in the contactor divided by the total volume that is occupied by the adsorbent particles This parameter characterizes the fraction of the bed volume in which the fluid moves ε = − (ρf /ρa ) 15 Adsorption Term Contactor or adsorber density, ρf Performance properties Adsorbate Breakthrough profile Carbon use rate Equilibrium isotherm Specific throughput Treatment objective Empty-bed contact time (EBCT) Definition Weight of the dry (and fresh) adsorbent particles divided by the total volume of the packed bed, including the bed pore volume Molecule that accumulates or adsorbs onto the adsorbent material Relationship between the adsorbate concentration leaving the adsorber as a function of the adsorber run time Mass of adsorbent used per volume of water treated to a given treatment objective Equilibrium partitioning relationship between the bulk aqueous-phase adsorbate concentration and the solid-phase adsorbate concentration at a constant temperature Volume of water treated per mass of adsorbent used at a given treatment objective Aqueous-phase adsorbate concentration that determines the bed life of a GAC adsorber or maximum value leaving a PAC contactor Volume of the bed occupied by the GAC (including voids) divided by the flow rate to the column Adsorption is a mass transfer operation in which substances present in a liquid phase are adsorbed or accumulated on a solid phase and thus removed from the liquid Adsorption processes are used in drinking water treatment for the removal of taste- and odor-causing compounds, synthetic organic chemicals (SOCs), color-forming organics, and disinfection by-product (DBP) precursors Inorganic constituents, including some that represent a health hazard, such as perchlorate, arsenic, and some heavy metals, are also removed by adsorption Reactions with granular activated carbon (GAC), a common adsorbent, can also be used to dechlorinate drinking water The primary adsorbent materials used in the adsorption process for drinking water treatment are powdered activated carbon (PAC) and GAC Powdered activated carbon is added directly to water and can be applied at various locations within a water treatment plant and is usually removed by sedimentation or filtration Granular activated carbon is usually employed after filtration just prior to postdisinfection and is operated in a fixedbed mode Granular activated carbon is also used in the upper layer of 1119 1120 15 Adsorption dual- or multimedium filters or as a substitute for conventional granular filter media The discussion that is presented in the following sections is intended to provide an introduction to adsorption processes and methods used for the design of PAC and GAC systems The topics discussed include (1) development of the adsorption phenomena; (2) manufacture, regeneration, and reactivation; (3) fundamentals of adsorption; (4) development of isotherms and equations used to describe adsorption equilibrium; (5) applications using PAC; and (6) applications using GAC 15-1 Introduction to Adsorption Phenomena To provide a perspective for the material to be presented in this chapter, the historical development of adsorption processes and present applications of adsorption materials in water treatment is discussed in this section Adsorption Phenomena The constituent that undergoes adsorption onto a surface is referred to as the adsorbate, and the solid onto which the constituent is adsorbed is referred to as the adsorbent During the adsorption process, dissolved species are transported into the porous solid adsorbent granule by diffusion and are then adsorbed onto the extensive inner surface of the adsorbent Dissolved species are concentrated on the solid surface by chemical reaction (chemisorption) or physical attraction (physical adsorption) to the surface Physical adsorption and chemisorption mechanisms are listed in Table 15-1 Physical adsorption is a rapid process caused by nonspecific Table 15-1 Comparison of adsorption mechanisms between physical adsorption and chemisorption Parameter Physical Adsorption Chemisorption Use for water treatment Most common type of adsorption mechanism Rare in water treatment Process speed Limited by mass transfer Variable Type of bonding Nonspecific binding mechanisms such as van der Waals forces, vapor condensation Specific exchange of electrons, chemical bond at surface Type of reaction Reversible, exothermic Typically nonreversible, exothermic Heat of adsorption 4–40 kJ/mol >200 kJ/mol 15-1 Introduction to Adsorption Phenomena 1121 binding mechanisms such as van der Waals forces and is similar to vapor condensation or liquid precipitation Physical adsorption is reversible, that is, the adsorbate desorbs in response to a decrease in solution concentration Physical adsorption is the most common mechanism by which adsorbates are removed in water treatment The physical adsorption process is exothermic with a heat of adsorption that is typically to 40 kJ/mol (about two times greater than the heat of vaporization or dissolution for gases and liquids, respectively) Chemisorption is more specific because a chemical reaction occurs that entails the transfer of electrons between adsorbent and adsorbate, and a chemical bond with the surface can occur The heat of adsorption for chemisorption is typically above 200 kJ/mol Chemisorption is usually not reversible, and desorption, if it occurs, is accompanied by a chemical change in the adsorbate What is commonly referred to as ‘‘irreversible adsorption’’ is chemisorption because the adsorbate is chemically bonded to the surface While physical adsorption and chemisorption can be distinguished easily at their extremes, some cases fall between the two, as a highly unequal sharing of electrons may not be distinguishable from the high degree of distortion of an electron cloud that occurs with physical adsorption (Adamson, 1982; Kipling, 1965; Satterfield, 1980) Because most water treatment applications involve physical adsorption, physical adsorption mechanisms are discussed in greater detail in this chapter Modern purification of water supplies by adsorption has a short history as compared to other processes, although the use of adsorption has been reported in a 4000-year-old Sanskrit text (Sontheimer et al., 1988) Adsorption was first observed in solution by Lowitz in 1785 and was soon applied as a process for removal of color from sugar during refining (Hassler, 1974) In the latter half of the nineteenth century, charcoal adsorbers (charcoal is not activated and contains underdeveloped pores) were used in U.S water treatment plants (Croes, 1883) The first GAC units for treatment of water supplies were constructed in Hamm, Germany, in 1929 and Bay City, Michigan, in 1930 In the 1920s, Chicago meat packers used PAC to remove taste and odor in water supplies that were contaminated by chlorophenols (Baylis, 1929) Powdered activated carbon was first used in municipal water treatment in New Milford, New Jersey, in 1930 and its use became widespread in the next few decades, primarily for taste and odor control During the mid-1970s, interest in adsorption as a process for removal of organics from drinking water was heightened because the public became increasingly concerned about water sources that were contaminated by industrial wastes, agricultural chemicals, and municipal discharges Another major concern was the formation of DBPs during chlorination of water containing background organic matter (referred to as DBP precursors) Historical Development 1122 15 Adsorption It has been found that activated carbon can be effective in removing some of the DBP precursors Applications of Adsorption Materials Three types of commercially available adsorbents merit consideration in water treatment: zeolites, synthetic polymeric adsorbents, and activated carbon Most activated carbons have a wide range of pore sizes and can accommodate large organic molecules such as natural organic matter (NOM) and synthetic organic compounds (SOCs) such as pesticides, solvents, and fuels Synthetic polymeric adsorbents usually have only micropores, which prevents them from adsorbing NOM Zeolites (aluminosilicates with varying Alto-Si ratios) tend to have very small pores, which will exclude some synthetic organic compounds Granular ferric hydroxide and iron-impregnated GACs have been developed to remove arsenic Ammonia-treated GAC has been use to increase the adsorption capacity of GAC for bromated and perchlorate, and it is likely that this would increase GAC adsorption capacity for other anionic species; however, there are no commercially available GACs Properties of several commercially available adsorbents are reported in Table 15-2 Porous adsorbents can have a large internal surface area (400 to 1500 m2 /g) and pore volume (0.1 to 0.8 mL/g) and as a result can have an adsorption capacity as high as 0.2 g of adsorbate per gram of adsorbent, Table 15-2 Properties of several commercially available adsorbents Adsorbent Manufacturer Type Surface Area, m2 /g (BET)a Packed Bed Density, g/cm Pore Volume, cm3 /g Filtrasorb 300 (8×30) Calgon GAC 950–1050 0.48 0.851 Filtrasorb 400 Calgon GAC 1075 0.4 1.071 CC-602 US Filter/Wastates Coconut-shell-based GAC 1150–1250 0.47–0.52 0.564 Aqua Nuchar MWV PAC 1400–1800 0.21–0.37 1.3–1.5 Dowex Optipore L493 Dow Polymeric >1100 0.62 1.16 Lewatit VP OC 1066 Bayer Synthetic polymer 700 0.5 0.65–0.8 a BET is the Brunauer, Emmett, and Teller method for measuring surface area based on gas (usually nitrogen) adsorption Source: Adapted from Sontheimer et al (1988), Crittenden (1976), Lee et al (1981), Munakata et al (2003), and Sigama_Aldrich Online Catalog (2004) 15-1 Introduction to Adsorption Phenomena 1123 depending on the adsorbate concentration and type Synthetic polymeric resins, zeolites, and activated alumina have been used in water treatment applications, but activated carbon is the most commonly used adsorbent because it is much less expensive than the alternatives Activated carbon is manufactured from natural, carbonaceous materials such as coal, peat, and coconuts by several inexpensive processes (e.g., high temperatures ∼800◦ C and steam) Consequently, most of the discussion in this chapter centers on the use of activated carbon; where appropriate, alternative adsorbents are discussed Activated carbon is available in essentially two particle size ranges: PAC (mean particle size 20 to 50 μm) and GAC (mean particle size 0.5 to mm) The principal uses, advantages, and disadvantages of using PAC versus GAC are reported in Table 15-3 At present, the applications of adsorption in water treatment in the United States are predominantly for taste and odor control In a 1984 survey, 29 percent of the water utilities used PAC (AWWA, 1986), and in a 1989 survey it was reported that 63 percent of the water plants used PAC and percent used GAC for taste and odor control (Suffet et al., 1996) Currently, it is thought that about 90 percent of the surface water treatment plants worldwide use PAC on a seasonable basis (Hansen, 1975; Sontheimer, 1976) In 1996, there were 300 GAC surface water plants and Table 15-3 Principal uses, advantages, and disadvantages of granular and powdered activated carbon Parameter Granular Activated Carbon (GAC) Powdered Activated Carbon (PAC) Principal uses ❑ Control of toxic organic compounds ❑ Seasonal control of taste and odor that are present in groundwater ❑ Barrier to occasional spikes of toxic organics in surface waters and control of taste and odor compounds compounds and strongly adsorbed pesticides and herbicides at low concentration (500 Å) Peat Wood Coal Coconut shell Material used to make activated carbon